A digester is a vessel in which wood chips are treated with heat, liquid, and chemicals in order to break down the chips into a pulp that can be further processed into paper, paperboard, or the like. The digester is typically an upright cylinder with a conically tapering or domed top end. A batch of wood chips may be loaded into the digester and then pulped, or wood chips may be continuously loaded in at the top end and pulp continuously removed at the bottom. The cooking liquor is continuously pumped into and out of the digester, so that its temperature and chemical content can be maintained.
In order to allow free circulation of the cooking liquor into and out of the digester, while keeping the chips within the digester, perforated metal screen plates are provided, spaced a short distance inward from the walls of the digester. These screen plates define and separate a central volume, containing the wood chips, and an annular space, acting as a manifold by means of which the liquor can be distributed and collected. The screen plates are typically made of stainless steel, and the perforations in the screen plates are typically circular holes.
For practical reasons, the screen typically consists of a circle of from 16 to 24 panels, each individually mounted on and removable from supports within the digester. The panels are usually curved so that they together form a cylinder or a truncated cone, depending on where in the digester they are to be installed. For example, in the conical top of a digester there may be 20 panels, each about 60″ (1.5 m) long, 6.5″ (165 mm) wide at the narrow end, and 20″ (500 mm) wide at the wide end, forming a cone about 40″ (1 m) in diameter at the narrow end and about 125″ (3.25 m) in diameter at the wide end. The panels may instead be rectangular or trapezoidal, forming a multi-sided prism or pyramid. These panels are easier to fabricate and, especially if the number of panels in the circle is large, form an adequate approximation to a cone or cylinder for many purposes.
As an alternative to holes, it has been proposed to use screen plates with straight slots angled obliquely. Examples of such screen plates are shown in U.S. Pat. No. 6,039,841 (Hernesniemi) and U.S. Pat. No. 6,165,323 (Shearer), both assigned to Ahlstrom.